(1) Field of the Invention
The present invention relates to a belt type cast sheet continuous caster (a so-called belt caster) for directly casting a sheet- or bar-like steel strip from a molten metal.
More specifically, the invention relates to an improvement of cooling systems arranged behind belts for supporting longitudinal side faces of a cast strip, and the invention is particularly directed to a proposal for a steel continuous casting technique which allows cooling without causing belt thermal deformation or melt leakage occurrence even when steel strips having different widths are produced.
(2) Related Art Statement
As converged type belt casters using a pair of circulatorily movable belts opposed to each other at a part of running paths thereof, there is a caster disclosed in Japanese Patent Laid-open No. 59-92,154 as shown in FIG. 2. According to this known belt caster, coolers so-called "cooling pads" having numerous water feed and discharge openings opened to movable (metal) belts are arranged behind the belts. The belt is cooled by forming a flowing water film of cooling water between the cooling pad and the belt.
In the belt type continuous caster shown in FIG. 2(a), a casting space is defined by opposed metal belts 1 and 2 which are circulated via plural guide rolls 32a,32b,32c,32d,32e and 32f and constitute longitudinal side faces while a gap is maintained therebetween over a specific distance for holding a molten steel or a solidified shell, and mold short side plates 3 and 4 brought into intimate contact with the metal belts 1 and 2 near the side edge portions thereof. In particular, since the diameter of a pouring nozzle 6 is not less than about 100 mm and the thickness of a cast sheet is not more than 50 mm, each of the mold short side plates 3 and 4 is designed in such a substantially inverse triangle with a wider upper portion that it becomes gradually narrower toward the lower portion and has a constant width at the lower portion. The short side plates have refractory lining layers 3a and 4a, respectively.
Further, as shown in FIG. 2(b), cooling pads 8 each having water fed openings 11 and water discharge openings 12 are provided behind the metal belts 1 and 2 as coolers. For instance, as shown in FIG. 2(c), water feed openings 11 of the cooling pads are provided in a row in a width direction of the cooling pad, and water discharge openings 12 are provided in the next row. In the same manner, rows of the water feed openings and rows of the water discharge openings are alternatively provided in the vertical direction of the cooling pads, that is, in the casting direction. While cooling water flowing out from the water feed openings 11 flows into the water discharge openings 12, a flowing water film F is formed between the metal belt 1 and the cooling pad 8 as shown in FIG. 2(d), thereby cooling and supporting the metal belt.
Apart from the above technique, the present inventors have make it possible to widely sue the belt caster through movably supporting side plates which support respective short side faces of the molten metal or the cast strip and thereby allowing change in the casting width and realizing the casting in different widths. However, there have occurred the following problems peculiar to the casting of the cast strips of different widths. The problems have been solved by the present invention.
1. Prevention of fins:
As shown in FIG. 3(a) an external force applied to the moving belt on the cooling pad side is divided according to (1) a zone (zone A) being in contact with the cast strip or the molten steel and (2) the opposite side zones of the movable belt (zone B) adjacent to the zone A on the basis of variations in tension, thermal stress, static pressure of the molten steel, etc. applied onto the movable belt, and both the zones are extremely different from each other. Therefore, the thickness of the water flow film is not uniform between both the above zones. Particularly, when the thickness of the flowing water film on the opposite side zones of the movable belt becomes smaller, a gap between the belt and the mold short side plate is largely opened, so that the molten steel may enter the gap to cause fins.
Assuming that a belt temperature difference between the zones A and B is taken at .DELTA.T(.degree.C.), belt tensions .sigma..sub.c and .sigma..sub.t in the zones A and B, respectively, due to thermal stress produced in the movable belt 1 during pouring are given by the following expressions, respectively: ##EQU1## in which .DELTA..sigma. is a stress produced by the thermal stress, and represented by the following equation( (3): EQU .DELTA..sigma.=.alpha..DELTA.TE (3)
and .sigma.o, B, b, .sigma., and E are an initial stage tension (kg/mm.sup.2) of the belt, the width (mm) of the belt, the width of a cast strip (mm), a coefficient of linear thermal expansion of the belt, and a Young's modulus of the belt, respectively.
According to the inventor's investigation, it was recognized that different functions are needed in the roles of the cooling pads 7, 8 between the flowing water film corresponding to the zone A and that corresponding to the zone B.
(i) The water film at the zone A has a function to cool and support a cast strip through the movable belt.
(ii) The water film at the zone B has a function to narrow a space between the side plate (mold short side plate) and the movable belt and prevent the melt leakage, at the zone where the side plate is present.
Further, the present inventors revealed through their investigation that when the cooling pad 7 is curved at the upper portion and is made flat at the lower portion, the cooling pad 7 is divided into the following three zones in the casting direction, as shown in FIG. 2a.
Zone 1: Zone wherein the pad face has a curvature R and the side plates are present in the side edge portions of the belt PA1 Zone 2: Zone where the pad face is flat and the side plates are present at the side edge portions of the belt PA1 Zone 3: Zone where the pad face is flat and no side plates are present at the side edge portions of the belt. PA1 P.sub.A : molten steel static pressure PA1 P.sub.B : 0 PA1 P.sub.A : molten steel static pressure PA1 P.sub.B : 0
In the zone 1, pressures P.sub.A and P.sub.B which the flowing water film is required to support at the zones A and B, respectively, are given by the following expressions (see FIG. 3(b) and FIG. 3(c)): ##EQU2## where h is the belt thickness.
Therefore, since P.sub.A and P.sub.B are not equal to each other and .sigma..sub.t &gt;&gt;.sigma..sub.c, P.sub.A is always smaller than P.sub.B.
In FIGS. 3(b) and (c) are schematically illustrated P.sub.A, P.sub.B and the pressure P.sub.W of the water flow film in the width direction thereof.
In the zone B, when P.sub.W &lt;P.sub.B, a melt leakage occurs between the side plates 3 and 4 and the moving belts 1.
In the zone A, when P.sub.W &gt;&gt;P.sub.A, the thickness of the water flow film becomes larger, so that the thermal deformation of the belt occurs due to the insufficient cooling capacity, and it causes the change in the thickness of the cast strip.
When P.sub.W &lt;&lt;P.sub.A, the flowing water film is cut to cause the thermal deformation of the belt.
As explained in the above, it was ultimately found out that when the flowing water film from the cooling pad is used for cooling and supporting the cast strip, it is necessary to provide difference in the pressure of the flowing water film in the belt width direction between opposite sides of near a portion where the side plates contacts with the molten steel.
On the other hand, the following measures (i) and (ii) are considered as means for affording difference in the pressure of the water film in the width direction.
(i) A water feed header portion inside the pad 7 is divided corresponding to the zones A and B by partition walls and cooling water is supplied to the zone B at a greater flow rate per unit width as compared with the zone A.
(ii) A water discharge header portion inside the pad 7 is divided corresponding to the zones A and B by partition walls and the flow path resistance of discharge water in the zone B is made larger as compared with the zone A.
However, the pad (cooler) having the fixed partition walls in the header as shown in FIG. 3(b) can cope with the casting of a cast strip having a specified width, but it suffers the following casting troubles when the width of the cast strip is varied:
(i) When the width of the cast strip is larger than the specified one, the pressure of the water film at a part of the zone A becomes higher, so that the thickness of the water film increases to make the shape of the cast strip poorer and/or produce the thermal deformation of the belt.
(ii) When the width of the cast strip is smaller than the specified one, the pressure of the water film at a part of the zone B becomes lower than the specified water film pressure, so that a gap is produced between the side plate and the movable belt to cause the fins.
In addition, since the cooling pad 7 is flat in the zone 2, the P.sub.A and P.sub.B become as follows:
However, since a force must be assured for pushing the movable belt to the side edge face of the side plates, the condition that the flowing water film pressure at the zone A is smaller than that at the zone B must be satisfied, and the conditions in the zone 1 must also be assured.
2. Prevention of unbalanced flow rate of cooling water:
Next, an external force applied to the belt in the zone 3 where the cooling pad is flat and no short side plate is present is:
FIG. 4 shows a conventional cooling system. When the casting width is wider (W), the water flow film in the liquid passing through a space between the movable belt and the cooling pad body receives substantially the equal internal pressure over the whole zone (a--a') in the width direction and thereby uniform cooling is carried out. However, when the casting strip is narrow (w), the internal pressure of the water film flowing inside the liquid-passing space differs between the locations a,a', and an interval from b to b' and a larger amount of cooling water flows through water feed openings at a,a', having a smaller flow path resistance, so that an amount of the cooling water necessary for cooling the cast strip is lacking at the b-b' interval as a main cooling section. Consequently, it is often seen that such leads to casting troubles such as deformation of the movable belt and the break-out.
Moreover, the conventional cooling system having the structure that the fixed partition walls 36a and 36b are provided in the water feed headers in the cooling pads 7 and 8 as shown in FIG. 3(b) can cope with only a case where the casting width is constant, but when the casting width is varied, such a cooling system has the defects that the thermal deformation of the movable belts, wasteful use of cooling water, and disorder of the cast strip shape occur.
(3) Prevention of belt thermal deformation:
In order to prevent the thermal deformation of the belts over the zones 1, 2 and 3, it is necessary to decrease .DELTA..sigma. shown by the above expression (3). In order to decrease the .DELTA..sigma., it is necessary to make the temperature different .DELTA.T between the zones A and B smaller. In order to make .DELTA.T smaller, it is effective that the water feed header sections to the zones A and B are isolated from each other, and a fluid having a temperature higher than that of the cooling fluid fed to the zone A is fed to the zone B.
The cooling pad in which the inside of the header is divided by the fixed partition walls as shown in FIG. 3(b) cannot cope with the different width casting, and has the shortcomings that when the steel strips with different widths are produced, casting troubles due to the thermal deformation of the belts, bad shape of the cast strip, and the fins occur.